Dwarf mutant of Xiphophorus helleri is used as the female parent to cross normal body type male, showing that dwarf individual can be identified in the offspring and such character is inheritable. The body type pattern of offspring exhibits a successive distribution, similar to male or female parent, in addition to intermediate type. It is less likely to discriminate the individual phenotype by using a clear definition. Dwarf mutants of Xiphophorus helleri manifest as short bloated body, clumsy swimming, and significantly altered trunk length-height ratio. This mutation retards fish growth and compromises the reproductivity. The self-cross and back-cross of F1 progenies also result in a successive distribution in body type among various crosses. The dwarf characteristic in X. helleri is not inherited in a simple alternative manner, which affects a group of characters rather than a single character. The consistency with primary feature of quantitative character suggests that such mutation may be controlled by multiple quantitative character genes. The skeletal morphological analysis indicates that the spinal length of X. helleri mutant is significantly shortened, whereas the number of vertebrae is comparable to that of normal individual, ranging from 26 to 29. The dramatically shortened vertebral body in the mutant impairs the longitudinal development of spine. The neural spine and hemal spine are in a tight arrangement,whose development is also significantly retarded. Therefore,the dwarf mutation in X. helleri is caused by the developmental defect in vertebral column, and the vertebral malformation underlies the body type alteration anatomically. As fish is representative of lower-ranking vertebrate,the dwarf swordtail mutant is reproducible,and the character is inheritable and closely associated with spinal development. Such mutant is a potential fish model for the systemical research.%以侏儒剑尾鱼为母本,与正常体型的雄性剑尾鱼进行交配,发现其后代的体型表现呈连续分布,部分与父本类似,部分与母本相仿,且存在中间类型,很难有明确的差别界限以区分个体表型的不同.后代中的侏儒个体主要表现为体型短,身体臃肿,游动笨拙,其体长和体高的比例出现了明显的变化.F1自交和回交,结果显示:不同组合后代的体型也呈连续分布.剑尾鱼的侏儒性状可遗传,但后代的性状表现不只是一种表型,而是影响一组表型,符合数量性状遗传的基本特征,推测或为多个数量性状基因控制.通过对突变剑尾鱼骨骼形态的分析发现:侏儒剑尾鱼的脊柱长度明显变短,但脊椎骨数量与正常个体并无差异,都在26-29之间;突变体的椎体明显变短,脊椎的纵向发育受阻,髓棘、脉棘等排列紧密,发育也受影响.因而推测,剑尾鱼的侏儒突变是由于脊椎的发育缺陷所致,椎体异常是其体型变化的结构基础.鱼类是低等脊椎动物的代表,剑尾鱼侏儒突变个体不仅可育,其性状也能遗传,且与脊椎的发育关系密切,或可作为模型材料进行系统研究.
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